Local tissue hyperosmolarity in contracting skeletal muscle resulting from accumulation of the products of metabolism has been suggested to be an important factor in producing the functional hyperemia of exercise. It is intended to define the role which skeletal muscle tissue osmolarity plays in the control of skeletal muscle blood flow in exercise by using a micro-cryoscopic technic directly at the muscle tissue site. If tissue hyperosmolarity is found, an attempt will be made to identify the substances involved. The degree of perfusion heterogeneity existing in skeletal muscle at rest and during exercise will be compared using the tritiated water technic recently developed in this laboratory. Using this same tritiated water technic, the changes in regional blood flow and, therefore, in the degree of perfusion heterogeneity in resting and exercising skeletal muscle resulting from infusion of a hypertonic blood perfusate will be determined.